SMILE: neural signal acquisition and intra-body transmission for facial nerve bypass, An acute feasibility study and proof-of-concept in a rat model

Facial paralysis is a disabling condition with severe functional and aesthetic consequences. Facial paralysis affects approximately 1.8% of individuals over their lifetime, with approximately 30% of affected patients developing persistent deficits; among these, patients with permanent flaccid paralysis and severe facial asymmetry do not resolve with pharmacological treatment and require surgical intervention. It is specifically this surgically relevant subgroup that represents the target population of the SMILE framework. The SMILE framework (bypaSs of a facial nerve lesion through intra-body biocoMpatIbLE communication technologies) validates the feasibility of the communication infrastructure required to establish a functional neural bypass link between the healthy side and a surgically reinnervated contralateral side of the face. This work presents a preliminary interdisciplinary experimental and engineering approach underlying the SMILE framework, combining neurophysiological validation in 15 adult Wistar rats with the design of ultra-low-power intra-body communication links based on galvanic coupling (GC) and ultrasound (US). Microsurgical cuff electrodes were implanted around the buccal branch of the healthy facial nerve to record odor-evoked motor outputs. ENG signals recorded from the intact buccal branch of the facial nerve on one side were transmitted to the contralateral side, across the animal's facehead. Engineering evaluations demonstrated robust transmission capabilities, with the GC link achieving a mean equivalent SNR of 18.2 ± 0.6 dB and a mean normalized cross-correlation of r = 0.72 ± 0.09 between transmitted and reconstructed ENG signals, with MSE on the order of 10−2. The US link achieved an equivalent SNR around 12 dB with MSE around 5 · 10−2, supporting the feasibility of intra-body neural signal relay through biological tissues.